1. Field of the Invention
The present invention relates to the inspection of fuel assemblies in nuclear power plants and, more specifically, to an arrangement for inspecting fuel rods of boiled water reactors using an ultrasonic transducer which does not require removal of the fuel channel slip encasing the fuel rods.
2. Description of the Related Art
Two types of nuclear fuel reactors are primarily used in the nuclear power industry: pressurized water reactors (PWR) and boiling water reactors (BWR). The nuclear fuel for either of these type of reactors is housed in fuel assemblies containing an array of fuel rods, each fuel rod in turn containing the fissionable material used to power the reactor. The fuel assemblies are submerged in circulating coolant water during operation, and are designed to efficiently transfer the generated fission heat to the coolant water.
The conventional technique for testing leakage from the fuel rods of both PWR and BWR reactors is to sip the water surrounding the fuel rods, and then test the water for radioactivity. Three different types of sipping methods exist: wet sipping, dry sipping and a more recent technique of vacuum sipping, to which U.S. Pat. No. 4,034,599 assigned to General Electric Co., is directed.
In a typical vacuum sipping cycle, the fuel assembly is inserted in an isolation chamber, which is then sealed and flushed with either demineralized or reclaimed water. In preparation for testing, a gas space is established above the fuel assembly by releasing a small quantity of water from the chamber while injecting air at the top at a slightly higher than pool pressure. This gas is then evacuated by a vacuum pump and passed through an in-line beta scintillation detector.
Although vacuum sipping is faster than either wet or dry sipping because sample acquisition and laboratory analysis steps are eliminated, all three sipping techniques merely detect the presence of fissionable material which has leaked into the coolant water. None of the sipping techniques isolate a problem to a particular fuel rod, and thus all require subsequent removal and testing of all fuel rods in the assembly.
In view of the above drawback, a subsidiary related to the assignee of the present application developed a failed fuel rod detection system for PWR reactors employing an ultrasonic transducer in the form of a two-fingered probe which traverses horizontally through each row of the rod array. The probe transmits an ultrasonic pulse and can detect and localize the presence of minor quantities of water inside each individual fuel rod from the received signal. The presence of water implies a through-wall defect in the particular fuel rod, indicating a fuel rod failure. This ultrasonic apparatus and technique, covered by U.S. Pat. No. 4,193,843 to Womack et al., issued Mar. 18, 1980, is much more accurate than sipping, and provides to the user a real time plot of the PWR fuel assembly showing the actual location and analysis of each of the rods. Thus, no fuel assembly dismantling is necessary for inspection/detection.
Although the above-described failed fuel rod detection system could conceivably be used to inspect all types of light water reactor assemblies (both PWR and BWR), the rod assemblies of BWR reactors are typically encased with a fuel channel slip, and thus inaccessible to a horizontal probe. It is possible to remove the fuel channel slip of BWR reactors to test the fuel rods, but such disassembly is highly undesirable. As shown in FIG. 1, the only entrance aperture for testing the fuel rods in BWR assemblies without removing the slip is at the bottom, through the nosepiece 2. However, the nosepiece has only about a 31/2 inch diameter opening, thus partially obscuring direct access to the outer fuel rods from beneath. An inverted tripod extending across this opening in the nosepiece makes access to the individual fuel rods even more difficult.